feat(WPN-1476): route Solana asset reads via migration stages

[ulissesferreira]

Explanation

Part of migrating the Solana assets-controller staged migration from snap-solana-wallet#641 into this monorepo (PR 4 of 6, Lane A / feature-flag path only).

Routes Solana fungible asset reads through AssetsProvider based on RemoteFeatureFlagController migration stages, following the Tron WPN-1497 pattern (#96) rather than a literal CoreAssetsAdapter class:

• Add shouldTrackSnapAssets, isSnapOwnedAsset (NFTs), and async mapControllerAsset (ATA-aware)
• Stage-aware routing in AssetsService:
  • Off → Snap adapter only
  • ReadAssetsControllerWithFallback → Core with Snap fallback
  • ReadAssetsControllerOnly+ → Core fungibles; Snap-owned NFTs always from Snap
• Gate Snap fungible fetch/saveMany when stage disables tracking
• Update AccountsSynchronizer and KeyringAccountMonitor for migration-stage behavior

References

• Migrated from: feat(WPN-1476): route Solana asset reads via migration stages (Lane A - PR 4a) snap-solana-wallet#641
• Jira: WPN-1476 / WPN-1652
• Stacks on: feat(WPN-1652): add Core messenger plumbing for Solana AssetsProvider #122
• Tron parallel: feat(tron-wallet-snap): route fungible reads through Core AssetsController #96
• Next: hardcode Core reads and remove migration flags (WPN-1476-hardcode-core-assets-adapter-flag)

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them